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I am having some difficulty understanding a few things about genetic recombination, in part because of confusion from different diagrams in books.

  1. First of all, I wanted to verify whether I have understood correctly: It seems to me that when homologous chromosomes (which at this point are as sister chromatids joined at the centromere) join during the zygotene/pachytene stage, only one of the chromatids from each homologous chromosome form synaptonemal complexes with one of the chromatids in the other chromosome. Is this correct? Then only one of each of the chromatids is involved in the DNA recombination...

    1. Leading on from my previous point, I think this would mean that in the final four zygotes that are formed, there are four different chromosome possibilities: the maternal chromosome (call it m) which has not had any recombination events; the maternal chromosome M whih has had recombination events with the paternal chromatid. Then the corresponding paternal chromatid p and chromatic which has undergone recombination event P. Is my understanding correct here?

    2. Also, I have seen in some diagrams there is a single chiasmata forming between the homologous-chromosome chromatids and in other diagrams I have seen more than one chiasmata. I think the one chiasmata picture is a simplification and there can in fact be more than 1 crossing over point between the chromatids of homologous chromosomes.

    3. Finally, if it is so that only one of each pf the sister chromatids is involved in recombination between homologous chromosomes, is there a a preference as to which chromatid this is, or is it random?

I realise that if both of the sister chromatids undergo these recombination events, then many of my questions would be redundant! However from the books I am looking at now, it seems that only one chromatid undergoes these crossings over...

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    $\begingroup$ I can't imagine point 1 being correct. I've taught out of many textbooks and have never encountered this. I can't think of a mechanism which would inform one chromatid that synapsis is occuring on the other and then prevent it from occuring on that chromatid. $\endgroup$ – bpedit Jan 23 '17 at 18:27

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